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A Three-Way Partnership at the Bottom of the Sea

In dense fields of seagrass that carpet coastal waters around the world, there's a three-way interaction keeping the ecosystem thriving. Two-shelled mollusks called bivalves, bacteria inhabiting the bivalves' gills, and seagrasses themselves all live symbiotically, new research reveals. The finding helps explain the long-standing puzzle of how seagrasses can survive in murky shoreline waters and offers insight into how scientists can better restore seagrass ecosystems, which are declining worldwide.

"I think it's a really exciting and interesting idea," says ecologist Jay Stachowicz of the University of California, Davis, who was not involved in the new study. "This is a type of interaction that people really hadn't considered before as being critical to seagrasses."

Often called marine nurseries, seagrass meadows harbor juvenile fish that spend their adult lives in coral reefs. Because of their shoreline locations and lush grasses, the meadows have dense layers of sediment and decaying organic material, a rich feeding ground for most ocean life. But the muddy deposits present a conundrum for the seagrasses: the bacteria responsible for breaking down the decaying matter emit high levels of sulfide, which should be toxic to the plants.

After noticing a high number of bivalves tucked in the roots of one seagrass ecosystem, marine ecologist Tjisse van der Heide of the University of Groningen in the Netherlands surveyed the frequency of bivalves in 83 seagrass meadows around the world. He suspected that bivalves, which are known to have gill-inhabiting bacteria which break down sulfides, may help seagrasses thrive. In 97% of tropical seagrass beds and 56% of seagrass beds in more temperate locations, bivalves are present, he found. "This made a lot of sense," he says, "because sulfide production is very temperature dependent. The levels are higher in tropical waters."

To flesh out the potential interaction, Van der Heide and his colleagues set up simulated environments in the lab, with controllable levels of sulfide and different combinations of seagrasses and bivalves. Over 5 weeks, the researchers discovered, seagrasses housed with bivalves grew almost twice as much as those contained alone. In addition, they discovered that the bivalves were healthier and hardier when they lived in close proximity to seagrasses, the team reports online today in Science. The observations, coupled with measurements of sulfide and oxygen in the environments, suggest that the gill bacteria in bivalves break down sulfide around the roots of the grasses, letting them grow. In turn, the seagrasses produce oxygen that gives the bivalves an extra boost in energy production.

"There are many conservation and restoration efforts that involve just moving seagrasses around," says Van der Heide. "But a few weeks, months, or a year later, the grasses are usually dead." The new study hints that moving bivalves along with the seagrasses might be the key to helping the grasses thrive.

More research is needed, however, says Stachowicz. "There's a big gap in scale from a tank outside your lab to the entire world," he says. "Filling in that gap with field-based experiments would be the next logical step." Further studies could analyze the varying role of bivalves in diverse seagrass beds, he notes.

The findings also may suggest a coevolution of the organisms, says marine biologist Emmett Duffy of the Virginia Institute of Marine Science in Gloucester Point. Bivalves could have provided a way, initially, for land-based grasses to invade shallow oceans without being poisoned by sulfide. "The benefit of the bacteria to the bivalves and the bivalves to the seagrasses provides a very plausible explanation for how both the seagrasses and the bivalves radiated evolutionarily and increased in range over a short time."